1. Field of the Invention
The present invention relates to a synchronous electric machine having auxiliary permanent magnets for an electric vehicle.
2. Description of the Related Art
When an electric vehicle with a synchronous electric machine having auxiliary permanent magnets is running on a downhill road, the electric machine is driven by the wheels and generates a high output voltage. In order to withstand such output voltage, the size of the electric machine and the electric power inverter thereof become large. The output voltage of this kind of electric machine is generally controlled by changing the armature reaction. In other words, the output voltage becomes maximum when the armature reaction cannot be changed. On the other hand, when the permanent magnets are exposed to the armature reaction, the permanent magnets are subject to irreversible demagnetization.
JP-A-6-351206 proposes a synchronous electric machine having permanent magnets which is aimed to solve the above problem. This electric machine is composed of a stator having a field coil as well as an armature coil and a rotor having permanent magnets on one side of the outer periphery thereof and magnetic cores on the other side. Thus, the magnetic flux supplied to the armature coil is controlled by the field coil to prevent generation of an abnormally high output voltage. However, because fluctuating magnetic flux of the stator passes through the core sheets of the stator, the iron loss is substantial, reducing in the efficiency of the electric machine.